UPDATED
Use regex instead, see this sample snippet to get the idea:
{literal}
jQuery.validator.addMethod("checkId", function(value, element) {
var id_val = $("#id_number").val();
var reg= new RegExp("^mu\\d{2}\\D+$","i");
if (reg.test(id_val)){
return true;
}else{
return false;
}
}, "Please enter valid ID e.g, MUXYZ001");{/literal}